-LRB- CNN -RRB- -- The deadliest Ebola outbreak in history continues to plague West Africa as leaders scramble to stop the virus from spreading .

Over the weekend , Liberian President Ellen Johnson Sirleaf closed most of the country 's borders . The few points of entry that are still open will have Ebola testing centers and will implement preventive measures , she said . The president also placed restrictions on public gatherings and ordered hotels , restaurants and other entertainment venues to play a five-minute video on Ebola safety .

`` No doubt the Ebola virus is a national health problem , '' Sirleaf said . `` It attacks our way of life , with serious economic and social consequences . As such we are compelled to bring the totality of our national resolve to fight this scourge . ''

As of July 20 , the World Health Organization had confirmed 224 cases of Ebola in Liberia , including 127 deaths . Overall , Ebola has killed at least 660 people in West Africa . There were 45 new cases reported in the region between July 18 and July 20 .

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Health officials are worried about the virus spreading to other countries in the region . This is the first such outbreak to hit West Africa . On Friday , a Liberian man with Ebola died in Lagos , Nigeria , Nigerian Health Minister Onyebuchi Chukwu said .

Patrick Sawyer arrived at Lagos ' airport on July 20 and was isolated at a local hospital after exhibiting common Ebola symptoms . He told officials he had no direct contact with anyone who had the virus .

Lagos State Health Commissioner Dr. Jide Idris said authorities are working to identify people who may have come in contact with the man on his flights . The process has been delayed , he said , because the airline has not provided the passenger lists for all three of the flights Sawyer took . Sawyer flew first to Ghana before he went to Togo and then switched planes to fly to Nigeria , according to the health commissioner .

`` In collaboration with the Federal Ministry of Aviation , I 've also put into motion mechanisms to track down all contacts at high risk , '' Idris said .

The state department has identified 59 people so far who came into contact with the man . Twenty have been tested for Ebola .

On Monday , Arik Air , one of Nigeria 's biggest airlines , suspended operations into the country 's capital , Monrovia , and another city called Freetown , according to AllAfrica.com .

It is unlikely the virus would spread on a plane unless a passenger were to come into contact with a sick person 's bodily fluids , according to the Centers for Disease Control and Prevention .

Meanwhile , American health care workers helping to fight this deadly disease abroad have put themselves at risk .

Dr. Kent Brantly , a 33-year-old Indianapolis resident , had been treating Ebola patients in Monrovia when he started to feel sick .

Brantly works with Samaritan 's Purse , a Christian international relief agency founded by evangelists Franklin Graham and Robert Pierce . He has been the medical director for the Ebola Consolidated Case Management Center in Monrovia and has been working to help Ebola patients since October .

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`` When the Ebola outbreak hit , he took on responsibilities with our Ebola direct clinical treatment response , but he was serving in a missionary hospital in Liberia prior to his work with Ebola patients , '' said Melissa Strickland , a spokeswoman for the organization .

After testing positive for Ebola , he went into treatment at a Samaritan 's Purse isolation center at ELWA Hospital in Paynesville City , Monrovia . His condition is rapidly deteriorating and Samaritan 's Purse has been working to evacuate him for better care , but Liberia is not allowing the evacuation , according to Samaritan 's Purse vice president of international relief , Ken Isaacs . The reason for the delay is unclear .

The CDC said the doctor 's family had been with him , but left for the United States before he became symptomatic ; as such it is highly unlikely that they caught the virus from him . Out of an abundance of caution they are on a 21-day fever watch , the CDC said .

Another American working with Samaritan 's Purse has also been infected . Nancy Writebol from Charlotte , North Carolina , works with Serving in Mission , or SIM . She and her husband used to work with orphans and other children who struggle with poverty-related issues but expanded their efforts to take on the complex medical problems in Monrovia .

She had teamed up with the staff from Samaritan 's Purse to help fight the Ebola outbreak in Monrovia when she got sick . She , too , is undergoing treatment .

A spokesperson for Samaritan 's Purse said there have been riots outside the clinic .

Another doctor who has played a key role in fighting the outbreak in Sierra Leone , Dr. Sheik Humarr Kahn , is sick . He is being treated by the French aid group Medecins Sans Frontieres -- also known as Doctors Without Borders -- in Kailahun , Sierra Leone , according to a representative of that agency .

Kahn had been overseeing treatment of Ebola patients in isolation units at Kenema Government Hospital , which is about 185 miles -LRB- 298 kilometers -RRB- east of the capital , Freetown .

Sierra Leone 's minister of health and sanitation called the doctor a national hero for the sacrifices he has made in trying to stop the outbreak .

Doctors and medical staff are particularly vulnerable to the virus because it spreads through exposure to bodily fluids from the infected . It can also spread through contact with an object contaminated by an infected person 's bodily fluids .

The disease is not contagious until symptoms appear . Symptoms of Ebola include fever , fatigue and headaches . They can appear two to 21 days after infection , meaning many who are sick do n't know it .

The early symptoms then can progress to vomiting , diarrhea , impaired kidney and liver function , and sometimes internal and external bleeding .

Leaders in the international health community characterize the Ebola epidemic as the `` deadliest ever . ''

While the World Health Organization has mobilized to fight the epidemic , it can be a difficult one to stop . It is so highly infectious that it typically kills 90 % of those who catch it . The death rate in this particular outbreak had dropped to roughly 60 % since it has been treated early in many cases . There is , however , no Ebola vaccination .

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With the announcement that Liberia is closing its borders , the country also instituted a new travel policy to inspect and test all outgoing and incoming passengers . The hope is that such measures will stop the spread of the virus .

There has never been a confirmed case of Ebola spreading to a developed country , said Kamiliny Kalahne , an epidemiologist with Doctors Without Borders .

`` This is because people generally transmit the infection when they are very sick , have a high fever and a lot of symptoms -- and in these situations , they do n't travel .

`` And even if they do get sick once they travel to a developed country , they will be in a good hospital with good infection control , so they are very unlikely to infect others , '' she said .

`` This is not the great plague , '' CNN 's Dr. Sanjay Gupta said after his trip to the region in the spring . `` But it is a pretty formidable killer . ''

`` The grim reality is it often kills so quickly , people do n't have time to spread it . ''

The CDC held a media briefing Monday to emphasize the fact that there is `` no significant risk in the U.S. '' for an Ebola outbreak . CDC officials said with the spread of Ebola to U.S. health care workers , there is a real need for `` vigilance '' to make sure workers are careful , get tested if there is any suspicion of illness , and that all sick travelers should remain isolated when returning from areas affected directly by the epidemic .

Stephan Monroe , CDC 's deputy director of the National Center for Emerging & Zoonotic Infectious Diseases , said the CDC is working with American health care providers to educate them about Ebola . The agency sent a notice Monday to remind doctors to take important steps to avoid the spread of the illness , including asking their patients about their travel history , particularly if they have traveled to West Africa in the last three weeks .

Watch : Dr. Gupta dispels Ebola myths

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Aid organization trying to evacuate doctor infected with Ebola

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Liberia closes its borders to stop Ebola from spreading

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This is the deadliest outbreak in the history of the disease